<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      for (;;) {
        let grade = Number(prompt('输入您的分数 输入114514退出'))
        if (grade ===114514) break
        if (grade >= 90) {
          alert('A')
        } else if (grade >= 80 && grade < 90) {
          alert('B')
        } else if (grade >= 70 && grade < 80) {
          alert('C')
        } else if (grade >= 60 && grade < 70) {
          alert('D')
        } else if (grade >= 0 && grade < 60) {
          alert('E')
        } else {
          alert('输入正确的分数')
        }
      }
    </script>
  </body>
</html>
